In typical current control systems using the technique of peak control, a current sensing transducer is used to sense the current in the output power bridge, and an analog comparator compares the sensed voltage with a reference voltage related to the desired current for the load. This system is just able to assess whether the current is above or below a certain threshold. In a power bridge, the sensing element is a resistor coupled in series between the low side switches and ground. In this application, the current information may not always be available. In fact, it may depend on the conditions of current recirculation.
Typically, a peak current control system lets the load current increase until it reaches a set threshold and then turns off the power stage. The current decays for a time that depends on the type of control that is implemented: Toff-constant or constant pulse width modulated (PWM) frequency, as typical examples. In FIG. 1, a Toff-constant mode is shown.
The advantage of this approach is the simplicity of the circuitry for implementation. A disadvantage is that the system is affected by systematic error caused by changes of the current ripple. These changes in the current ripple notably increase with the absolute value of the current and are proportionally tied to the PWM period. The publication “Predictive Digital Current Programmed Control,” Jingquan Chen, et al., January 2003, Power Electronics, IEEE Transactions, Vol. 18, Issue 1, Part 2, Pages 411-419 discloses a predictive digital current control for “valley,” “peak,” or “average current” in basic converters topologies: buck, boost, and buck-boost. The publication reveals that for each parameter of interest, there is a choice of the most appropriate PWM for accomplishing a predictive digital current control that avoids oscillation problems. The optimal duty cycle is computed by the controller, on the basis of a number of electrical parameters, such as load current, input voltage, output voltage and inductance value, implying multiplications, and divisions.